In 1999, Kuperberg co-authored with Pavel Valtr a paper on “Almost-tiling the plane by ellipses” in Discrete Comput. Geom.22. Valtr had just a few years earlier co-authored with Erdős a paper on the Ramsey remainder in European J. Combin.19. Thus, Kuperberg’s Erdős number (http://planetmath.org/ErdHosNumber) is 2.